This invention relates to a semiconductor memory device having a mask ROM and rewritable PROM and more particularly to a semiconductor memory device in which data stored in a PROM can be apparently erased and rewritten.
In a conventional mask ROM, data is written in the process of manufacturing the mask ROM and it is impossible for the user to rewrite data after it was manufactured.
In the conventional mask ROM, if a data error due to bugs or the like in the program on the user side is found after the user bought the mask ROM, the mask ROM must be discarded.
However, the data error includes only approximately several bits in many cases, and if the error data of several bits can be corrected, it becomes possible to use the mask ROM without discarding the same.
Therefore, as is disclosed in Japanese Patent Application No. 7-320182, conventionally, it is considered to form memory cells in which the user can write data in addition to the mask ROM on a chip so as to compensate for bugs of several bits in the mask ROM.
In this case, it is sometimes necessary to rewrite data in the same address after corrected data is written into the memory cell. Therefore, it is desired that the compensation memory cell is formed of an erasable memory cell such as an EEPROM.
However, the manufacturing process of the erasable memory cell is more complicated than that of the mask ROM and the manufacturing cost is raised.